High temperature cobalt-based and nickel-based superalloys are commonly used in the manufacture of gas turbine engine components, including combustors, rotating blades and stationary vanes. During the operation of these components in the harsh operating environment of a gas turbine, various types of damage and deterioration of the components may occur. For example, the surface of a component may become cracked due to thermal cycling or thermo-mechanical fatigue or it may be eroded as a result of impacts with foreign objects and corrosive fluids. Furthermore, such components may be damaged during manufacturing operations even prior to entering service. Because the cost of gas turbine components made of cobalt-base and nickel-base superalloys is high, repair of a damaged or degraded component is preferred over replacement of the component.
Several repair techniques have been developed for various applications of superalloy materials. Fusion welding of superalloy materials is known to be a difficult process to control due to the tendency of these materials to crack at the area of the weld deposit/joint. However, with careful pre-weld and post-weld stress relief, control of welding parameters, and selection of welding materials, repair welds can be performed successfully on superalloy components.
Brazing is also commonly used to join or to repair superalloy components. One limitation of brazing is that brazed joints are typically much weaker than the base alloy, and so they may not be appropriate in all situations, such as repairs on the most highly stressed areas of the component.
Another process that has been used successfully for repair and material addition to superalloy components is known by several different names: diffusion bonding; diffusion brazing; and liquid phase diffusion sintering. These names generally refer to a process wherein a powdered alloy is melted at a temperature that is less than the liquidous temperature of the component alloy and allowed to solidify to become integral with the component. The powdered alloy typically includes particles of a high strength base alloy, for example the same alloy as is used to form the base component, along with particles of a braze alloy having a lower melting temperature than the base alloy, for example the base alloy with a melting point depressant such as boron or silicon. The following United States patents describe such processes and are hereby fully incorporated by reference herein: U.S. Pat. Nos. 4,381,944; 4,493,451; 5,549,767; 4,676,843; 5,086,968; 5,156,321; 5,437,737; 6,365,285; and 6,454,885. The component and powder are subjected to a heat cycle, often called a brazing heat treatment, wherein the temperature is selected so that the braze alloy having the lower melting temperature will become liquid and will wet the surfaces of the higher melting temperature base alloy and component alloy. The component is held at this elevated temperature for a sufficient interval to promote liquid phase sintering. Liquid phase sintering is a process whereby adjacent particles in a powder mass are consolidated by diffusion through a liquid phase present between the particles. As the melting point depressant diffuses away from the braze area, the melting point of the remaining material will increase and the liquid material will solidify to form the desired braze joint. This process may be used to join two pieces, to repair a damaged area, or to add material to a component. Upon completion of this cycle, typical braze alloys will have formed undesirable large blocky or script-like brittle phases composed of chromium, titanium, and the family of refractory elements (e.g., tungsten, tantalum) combined with the melting point depressants. These brittle phases weaken the repaired component and decrease its ductility in the region of the repair. A further post-braze diffusion heat treatment may be applied at a somewhat lower temperature to break down the brittle borides, carbides and silicides into fine, discrete blocky phases and to further drive the melting point depressant away from the braze joint to more fully develop the desired material properties. This diffusion heat treatment step may extend for a just a few hours to as long as 24 hours or more. Such a liquid phase diffusion bonding process is capable of forming a joint with material properties approximating but typically not as good as those of the base alloy. Welding is generally avoided proximate the braze joint because the embrittling effect of the residual melting point depressant may cause cracking during cool down from the high temperature required for welding. Further reductions in processing times and improvements in the material properties resulting from a diffusion brazing process are desired.